Priority call routing

ABSTRACT

A method includes receiving a priority message in an Internet Protocol Multimedia Subsystem (IMS) and identifying, via the IMS, resource availability through a first network and a second network. The method further includes routing, via the IMS, the priority message through one of the first network or the second network based on the identified resource availabilities of the first network and the second network.

BACKGROUND INFORMATION

The Internet Protocol Multimedia Subsystem (IMS) provides mobile andfixed multimedia services. The aim of IMS is not only to provide newservices, but all the services, current and future, that the Internetprovides. In this way, IMS gives network operators and service providersthe ability to control and charge for each service. In addition, usersare given the ability to execute services from their home networks, aswell as when the users are roaming.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary overview of implementations consistentwith principles of the invention;

FIG. 2 illustrates an exemplary system in which systems and methods,consistent with principles of the invention, may be implemented;

FIG. 3 illustrates an exemplary configuration of the user device of FIG.2;

FIG. 4 illustrates an exemplary functional block diagram of a portion ofthe Serving Call Session Control Function (S-CSCF) of FIG. 2;

FIG. 5 illustrates a flow chart of exemplary processing for obtainingresource availability information in an exemplary implementationconsistent with principles of the invention;

FIG. 6 illustrates a flow chart of exemplary processing for handlingpriority calls in an exemplary implementation consistent with principlesof the invention; and

FIGS. 7 and 8 illustrate examples of the processing described withrespect to FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description of implementations consistent withprinciples of the invention refers to the accompanying drawings. Thesame reference numbers in different drawings may identify the same orsimilar elements. Also, the following detailed description does notlimit the invention. Instead, the scope of the invention is defined bythe appended claims and their equivalents.

Implementations described herein relate to priority call routing in anInternet Protocol (IP) Multimedia Subsystem (IMS). FIG. 1 illustrates anexemplary overview 100 of implementations consistent with principles ofthe invention. As illustrated, a user device may place a priority call,such as a 9-1-1 call. The IMS may receive a message from the userdevice. IMS may identify that the message is a high priority message(e.g., based on information in the header of the message). Uponidentifying the message as a high priority message, the IMS maydetermine resource availability through the data network and the PublicSwitched Telephone Network (PSTN) for establishing the call as quicklyas possible to a Public Safety Answering Point (PSAP). If, for example,the resource utilization through the data network exceeds a thresholdand the resource utilization through the PSTN does not exceed thethreshold, the IMS may establish the call through the PSTN to the PSAPsince the call is likely to be established more quickly through thePSTN. If, on the other hand, the resource utilization through the PSTNexceeds a threshold and the resource utilization through the datanetwork does not exceed the threshold, the IMS may establish the callthrough the data network to the PSAP since the call is likely to beestablished more quickly through the data network. In some situations,the resource utilization through the data network may exceed thethreshold and the resource utilization through the PSTN may exceed thethreshold. In those situations, the IMS may establish the call through abackup emergency wireline link or a backup emergency wireless link.

FIG. 2 illustrates an exemplary system 200 in which systems and methods,consistent with principles of the invention, may be implemented. Asillustrated, system 200 may include a user device 210, an IMS 220, aResource Admission and Control Function (RACF) 230, an access network240, a core network 250, a data network 260, a PSTN gateway (GW) 270, aPSTN 280, and PSAPs 290. The number of components illustrated in FIG. 2is purely exemplary. A typical system 200 may include more or fewer userdevices 210, IMSs 220, RACFs 230, access networks 240, core network 250,data networks 260, PSTN gateways 270, PSTNs 280, and PSAPs 290 thanillustrated in FIG. 2. Moreover, system 200 may include other componentsthan illustrated in FIG. 2 that aid in establishing calls.

User device 210 may include one or more devices capable of establishinga telephone call (also called a session) with another device, such as aPSAP 290. For example, user device 210 may include a Session InitiationProtocol (SIP) telephone, a type of computer system, such as amainframe, minicomputer, or personal computer, and/or some other devicecapable of placing telephone calls.

IMS 220 may include a group of components for providingtelecommunications services. As illustrated, IMS 220 may include a ProxyCall Session Control Function (P-CSCF) 222, a Serving Call SessionControl Function (S-CSCF) 224, and a Breakout Gateway Control Function(BGCF) 226. IMS 220 may include other components than illustrated inFIG. 2.

P-CSCF 222 may receive session requests from user devices 210 located atboth public IP networks and private IP networks. P-CSCF 222 may validaterequests, forward the requests to selected destinations, and process andforward the responses.

S-CSCF 224 may be viewed as the brain of IMS 220. S-CSCF 224 may performsession control, registration, and service invocation for users of IMS220. S-CSCF 224 may receive a user profile from a home subscriber serverand route sessions requested by an IMS user (also called an “IMSsubscriber” or “subscriber”) associated with a user device 210. In oneimplementation, S-CSCF 224 may provide priority call handling based onresource availability information.

BGCF 226 may connect IMS-initiated calls to Circuit Switch (CS)customers (e.g., an IMS user calling a telephone number in the PSTN).BGCF 226 may, based on a routing policy, select an appropriate networkin which the connection is to be made or may select the appropriategateway to the other network. In one implementation, BGCF 226 may obtainresource availability information for, for example, PSTN 280 and providethe resource availability information to S-CSCF 224.

RACF 230 may include components that manage transport resources withinaccess and/or core networks, such as networks 240 and 250, and atnetwork boundaries, such as at edge nodes, session border gateways, etc.As illustrated, RACF 230 may include a Policy Decision Function (PDF)232, an Access Transport Resource Control (A-TRC) function 234, and aCore Transport Resource Control (C-TRC) function 236. RACF 230 mayinclude other components than illustrated in FIG. 2.

PDF 232 may provide service-based local policy control. PDF 232 mayauthorize resources based on policy and resource availability. In oneimplementation, PDF 232 may obtain resource availability informationfor, for example, networks 240, 250, and/or 260 and provide the resourceavailability information to S-CSCF 224,

A-TRC 234 may collect and maintain transport resource status informationfor devices within access network 240. Moreover, A-TRC 234 may collectand maintain topology information for access network 240. A-TRC 234 mayalso configure transport resources in access network 240.

C-TRC 236 may collect and maintain transport resource status informationfor devices within core network 250. Moreover, C-TRC 236 may collect andmaintain topology information for core network 250. C-TRC 236 may alsoconfigure transport resources in core network 250.

Access network 240 may include one or more networks through which userdevice 210 accesses a service provider's core network, such as corenetwork 250. Core network 250 may include one or more networksassociated with a service provider. Data network 260 may include one ormore networks, such a local area network (LAN), a wide area network(WAN), an intranet, the Internet, or a combination of networks.

PSTN gateway 270 may convert traffic between a data network, such asaccess network 240 or core network 250, and PSTN 280. In oneimplementation consistent with principles of the invention, PSTN gateway270 may convert SIP messages to PSTN signaling and convert PSTNsignaling to SIP messages. PSN 280 may include one or morecircuit-switched networks that transport signaling and/or content (e.g.,voice data) traffic.

PSAPs 290 may include one or more devices for receiving and processingemergency calls (e.g., 9-1-1 calls). For example, PSAPs 290 may includea public entity having personnel (e.g., operators or call takers) and/orequipment for initially answering or fielding incoming 9-1-1 calls.PSAPs 290 can be located in the vicinity of the source of the emergencycall and in the vicinity of emergency service providers to which thecall can be forwarded. In one implementation, a first PSAP 290 may beprovided for handling calls from data network 260 and a second PSAP 290may be provided for handling calls from PSTN 280. In otherimplementations, a single PSAP 290 may be provided for handling callsfrom both data network 260 and PSTN 280.

FIG. 3 illustrates an exemplary configuration of user device 210 in animplementation consistent with the principles of the invention. S-CSCF224 may be similarly configured.

As illustrated, user device 210 may include a bus 310, processing logic320, a memory 330, an input device 340, an output device 350, and acommunication interface 360. It will be appreciated that user device 210may include other components (not shown) that aid in receiving,transmitting, and/or processing data. Moreover, it will be appreciatedthat other configurations are possible.

Bus 310 may permit communication among the components of user device210. Processing logic 320 may include any type of processor ormicroprocessor that interprets and executes instructions. In otherimplementations, processing logic 320 may be implemented as or includean application specific integrated circuit (ASIC), field programmablegate array (FPGA), or the like. Memory 330 may include a random accessmemory (RAM) or another type of dynamic storage device that storesinformation and instructions for execution by processing logic 320, aread only memory (ROM) or another type of static storage device thatstores static information and instructions for processing logic 320,and/or some other type of magnetic or optical recording medium and itscorresponding drive for storing information and/or instructions.

Input device 340 may include a device that permits a user to inputinformation into user device 210, such as a keyboard, a keypad, a mouse,a pen, a microphone, one or more biometric mechanisms, and the like.Output device 350 may include a device that outputs information to theuser, such as a display, a printer, a speaker, etc.

Communication interface 360 may include any transceiver-like mechanismthat enables user device 210 to communicate with other devices and/orsystems. For example, communication interface 360 may include mechanismsfor communicating with PSAP 290 via one or more networks.

As will be described in detail below, user device 210 may allow a userto place a priority call, such as an emergency 9-1-1 call, a GovernmentEmergency Telecommunications Service (GETS) call, etc. User device 210may perform these and other acts in response to processing logic 320executing software instructions contained in a computer-readable medium,such as memory 330. A computer-readable medium may be defined as one ormore memory devices and/or carrier waves. The software instructions maybe read into memory 330 from another computer-readable medium or fromanother device via communication interface 360. The softwareinstructions contained in memory 330 may cause processing logic 320 toperform processes that will be described later. Alternatively, hardwiredcircuitry may be used in place of or in combination with softwareinstructions to implement processes consistent with the principles ofthe invention. Thus, systems and methods consistent with the principlesof the invention are not limited to any specific combination of hardwarecircuitry and software.

Priority determination logic 410 may receive a message (e.g., a SIPmessage) from user device 210 and determine whether the message is ahigh priority message, such as a 9-1-1 message, a GETS message, etc. Insome implementations, priority determination logic 410 may determinewhether the message is a high priority message based on informationstored in the header of the message. For example, in one implementation,priority determination logic 410 may determine that a message (e.g., aSIP message) is a high priority message based on a Resource Priorityheader field of the message. Other header fields may alternatively beused for indicating that a message is a high priority message.

Resource availability determination and storage logic 420 may receiveand store resource availability information from devices in system 200.For example, in one implementation, resource availability determinationand storage logic 420 may receive resource availability information fromBGCF 226 and PDF 232. The resource availability information from BGCF226 may include information regarding the availability of resourcesthrough PSTN 280. The resource availability information from PDF 232 mayinclude information regarding the availability of resources throughaccess network 240, core network 250, and/or data network 260. Resourceavailability determination and storage logic 420 may store the resourceavailability information received from BGCF 226 and PDF 232.

Priority routing logic 430 may route priority messages to a PSAP 290 viaa path determined based on the resource availability information storedby resource availability determination and storage logic 420. Forexample, priority routing logic 430 may determine whether to route apriority message through data network 260 or PSTN 280. In oneimplementation, priority routing logic 430 may select data network 260or PSTN 280 by comparing the resources available through data network260 to a threshold and comparing the resources available through PSTN280 to the threshold. The threshold may be set by a systemadministrator. Moreover, a different threshold may be associated withdata network 260 and PSTN 280 in some implementations.

FIG. 5 illustrates a flow chart of exemplary processing for obtainingresource availability information in an exemplary implementationconsistent with principles of the invention. In one implementation, theprocessing described in FIG. 5 may be implemented by S-CSCF 224. Inanother implementation, the processing may be implemented by anotherdevice in system 200.

Processing may begin with S-CSCF 224 receiving resource availabilityinformation from BGCF 226 and/or PDF 232 (block 510). As indicatedabove, the resource availability information from BGCF 226 may includeinformation regarding the availability of resources through PSTN 280 andthe resource availability information from PDF 232 may includeinformation regarding the availability of resources through accessnetwork 240, core network 250, and/or data network 260. S-CSCF 224 mayreceive the resource availability information from BGCF 226 and/or PDF232 via, for example, a Publish, Subscribe, Notify messaging procedure.For example, S-CSCF 224 may use a {Subscribe, Notify} technique tocapture the {Published} state of BGCF 226 and/or PDF 232 via SIP instantmessages (IMs). Other techniques for obtaining resource availabilityinformation from BGCF 226 and/or PDF 232 may alternatively be used. Inother implementations, S-CSCF 224 may receive resource availabilityinformation from devices in addition to or other than BGCF 226 and PDF232.

The resource availability information from BGCF 226 and/or PDF 232 maybe stored (block 520). For example, S-CSCF 224 may store the resourceavailability information in a memory, such as memory 330. In oneimplementation, S-CSCF 224 may associate information identifying thedevice, from which resource availability information is received, in thememory with the received information. S-CSCF 224 may continually updatethe stored resource availability as new resource availabilityinformation is received from BGCF 226 and PDF 232.

FIG. 6 illustrates a flow chart of exemplary processing for handlingpriority calls in an exemplary implementation consistent with principlesof the invention. The processing of FIG. 6 may be performed by S-CSCF224. The processing of FIG. 6 may be performed by another device insystem 200 in other implementations.

Processing may begin with S-CSCF 224 receiving a message from a userdevice, such as user device 210 (block 610). In one implementation, themessage may include a SIP message, such as an INVITE message or anothertype of message.

S-CSCF 224 may determine whether the message includes a high priorityindication (block 620). User device 210 may transmit a message with ahigh priority indication when, for example, a user of user device 210places an emergency communication, such as, for example, a 9-1-1 call, aGETS communication, or another type of high priority (or emergency)communication. S-CSCF 224 may identify that a message includes a highpriority indication based on a header of the message. In oneimplementation, S-CSCF 224 may determine that a message is a highpriority message when a Resource-Priority header field (or other headerfield) is set in the message (e.g., when the Resource-Priority headerfield includes a “1” bit).

If the received message does not include a high priority indication(block 620—NO), S-CSCF 224 may forward the message toward itsdestination according to typical routing procedures (block 630). Forexample, S-CSCF 224 may forward the message toward another IMSsubscriber, a PSTN destination, etc. in a known manner.

If, on the other hand, the received message includes a high priorityindication (block 620—YES), S-CSCF 224 may determine the resourceavailability for establishing the emergency communication with a PSAP290 through a PSTN, such as PSTN 280, and through a data network, suchas data network 260 (block 640). For example, S-CSCF 224 may determinethe resource availability based on information received from otherdevices in system 200, such as BGCF 226 and PDF 232. In oneimplementation, S-CSCF 224 may determine the resource availabilityinformation via a lookup operation.

S-CSCF 224 may compare the resource availability information for datanetwork 260 and the resource availability information for PSTN 280 to aresource availability threshold (block 650). Alternatively, S-CSCF 224may compare the resource utilization in data network 260 and theresource utilization in PSTN 280 to a resource utilization threshold.Each threshold may be configurable.

S-CSCF 224 may forward the received message to a PSAP 290 through PSTN280 or through data network 260 based on the resource availability ofPSTN 280 and data network 260 (block 660). For example, if the resourceavailability through PSTN 280 does not exceed the resource availabilitythreshold (or, in another implementation, the resource utilization ofPSTN 280 exceeds the resource utilization threshold) and the resourceavailability through data network 260 exceeds the resource availabilitythreshold (or, in another implementation, the resource utilization ofdata network 260 does not exceed the resource utilization threshold),S-CSCF 224 may select data network 260 instead of PSTN 280 forforwarding the received message to a PSAP 290. If on the other hand, theresource availability through PSTN 280 exceeds the resource availabilitythreshold (or, in another implementation, the resource utilization ofPSTN 280 does not exceed the resource utilization threshold) and theresource availability through data network 260 does not exceed theresource availability threshold (or, in another implementation, theresource utilization of data network 260 exceeds the resourceutilization threshold), S-CSCF 224 may select PSTN 280 instead of datanetwork 260 for forwarding the received message to a PSAP 290. In thisway, for example, the probability of establishing the emergencycommunication as quickly as possible is provided.

In those situations where the resource availability through PSTN 280 anddata network 260 exceed the resource availability threshold (or, inanother implementation, the resource utilization in PSTN 280 and datanetwork 260 do not exceed the resource utilization threshold), S-CSCF224 may randomly select PSTN 280 or data network 260 for forwarding thereceived message to a PSAP 290. In another implementation, S-CSCF 224may be configured to select either PSTN 280 or data network 260 when theresource availability through PSTN 280 and data network 260 exceed theresource availability threshold (or, in another implementation, theresource utilization in PSTN 280 and data network 260 do not exceed theresource utilization threshold).

In those situations where the resource availability through PSTN 280 anddata network 260 do not exceed the resource availability threshold (or,in another implementation, the resource utilization in PSTN 280 and datanetwork 260 exceed the resource utilization threshold), S-CSCF 224 mayuse a backup emergency wireline or wireless link for establishing theemergency communication with PSAP 290.

The following examples illustrate the processing described above withrespect to FIG. 6. In a first example 700, assume that a user of userdevice 210 places a 9-1-1 call, which causes a high priority message tobe transmitted to P-CSCF 222 of IMS 220 (block 710), as illustrated inFIG. 7. P-CSCF 222 may forward the high priority message to S-CSCF 224(block 720). In response to receiving the high priority message, S-CSCF224 may identify the message as a high priority message based, forexample, on a Resource-Priority header field being set in the message.In response to the message being identified as a high priority message,S-CSCF 224 may identify the resource utilization through PSTN 280 anddata network 260. Assume, in this example, that S-CSCF 224 determinesthat the resource utilization via data network 260 exceeds a resourceutilization threshold and that the resource utilization via PSTN 280does not exceed the resource utilization threshold. In this case, S-CSCF224 may forward the message to BGCF 226 (block 730). BGCF 226 mayforward the message to PSAP 290 via PTSN gateway 270 and PSTN 280 in aknown manner (blocks 740 and 750).

In a second example 800, assume that a user of user device 210 places a9-1-1 call, which causes a high priority message to be transmitted toP-CSCF 222 of IMS 220 (block 810), as illustrated in FIG. 8. P-CSCF 222may forward the high priority message to S-CSCF 224 (block 820). Inresponse to receiving the high priority message, S-CSCF 224 may identifythe message as a high priority message based, for example, on aResource-Priority header field being set in the message. In response tothe message being identified as a high priority message, S-CSCF 224 mayidentify the resource utilization through PSTN 280 and data network 260.Assume, in this example, that S-CSCF 224 determines that the resourceutilization via PSTN 280 exceeds a resource utilization threshold andthat the resource utilization via data network 260 does not exceed theresource utilization threshold. In this case, S-CSCF 224 may forward themessage to RACF 230 (block 830). RACF 230 may forward the message toPSAP 290 via core network 250 and data network 260 in a known manner(block 840).

CONCLUSION

Implementations described herein provide priority call routing in anInternet Protocol (IP) Multimedia Subsystem (IMS). Priority calls arerouted through a data network or PSTN based on resource utilization inthe data network and PSTN.

In the preceding specification, various preferred embodiments have beendescribed with reference to the accompanying drawings. It will, however,be evident that various modifications and changes may be made thereto,and additional embodiments may be implemented, without departing fromthe broader scope of the invention as set forth in the claims thatfollow. The specification and drawings are accordingly to be regarded inan illustrative rather than restrictive sense.

For example, while series of acts have been described above with respectto FIGS. 5 and 6, the order of the acts may differ in otherimplementations consistent with principles of the invention. Moreover,non-dependent acts may be performed in parallel.

It will be apparent that aspects of the embodiments, as described above,may be implemented in many different forms of software, firmware, andhardware in the implementations illustrated in the figures. The actualsoftware code or specialized control hardware used to implement theseembodiments consistent with the principles of the invention is notlimiting of the invention. Thus, the operation and behavior of thepreferred embodiments of the invention were described without referenceto the specific software code it being understood that software andcontrol hardware may be designed to implement the embodiments based onthe description herein

Further, certain portions of the invention may be implemented as “logic”that performs one or more functions. This logic may include hardware,such as an application specific integrated circuit, a field programmablegate array, a processor, or a microprocessor, software, or a combinationof hardware and software.

No element, act, or instruction used in the description of the presentapplication should be construed as critical or essential to theinvention unless explicitly described as such. Also, as used herein, thearticle “a” is intended to include one or more items. Where only oneitem is intended, the term “one” or similar language is used. Further,the phrase “based on” is intended to mean “based, at least in part, on”unless explicitly stated otherwise.

1. A method comprising: receiving, at an Internet Protocol MultimediaSubsystem (IMS), a message to request a communication through a packetswitched network, where the packet switched network includes a corenetwork associated with the IMS and an access network that providesaccess to the core network; determining, by the IMS, whether a senderdesignated the message as a priority message; when the sender designatesthe message as the priority message: identifying, by the IMS and inresponse to determining that the sender designated the message as thepriority message, a first resource availability through the packetswitched network and a second resource availability through a circuitswitched network, where identifying the first resource availabilityincludes: determining, via a resource access control function (RACF)associated with the IMS, resource availability and topology informationfor the access network, determining, via the RACF, resource availabilityand topology information for the core network, and determining the firstresource availability based on the resource availability and topologyinformation for the access network and the resource availability andtopology information for the core network, and establishing, by the IMS,the communication through one of the packet switched network or thecircuit switched network based on the first resource availabilitythrough the packet switched network and the second resource availabilitythrough the circuit switched network; and when the sender does notdesignate the message as the priority message, establishing thecommunication through the packet switched network without identifyingthe first resource availability through the packet switched network andthe second resource availability through the circuit switched network.2. The method of claim 1, where determining whether the sender hasdesignated the message as the priority message comprises: identifyingthe message as the priority message based on a header of the message. 3.The method of claim 1, where the message includes a Session InitiationProtocol (SIP) message, and where determining whether the sender hasdesignated the message as the priority message comprises: identifyingthe message as the priority message based on a Resource-Priority headerfield of the SIP message.
 4. The method of claim 1, where identifyingthe first resource availability and the second resource availabilityincludes: identifying the first resource availability based on firstresource availability information received from a first deviceassociated with the packet switched network, and identifying the secondresource availability based on second resource availability informationreceived from a second device that is associated with the circuitswitched network, where the first device and the second device differ.5. The method of claim 1, where establishing the communication throughthe one of the packet switched network or the circuit switched networkfurther includes: establishing the communication to a Public SafetyAnswering Point via the one of the packet switched network or thecircuit switched network.
 6. The method of claim 1, where identifyingthe first resource availability and the second resource availability androuting the message through one of the packet switched network or thecircuit switched network are performed by a Serving Call Session ControlFunction within the IMS.
 7. The method of claim 1, where establishingthe communication through one of the packet switched network or thecircuit switched network includes: comparing, prior to establishing thecommunication, the first resource availability through the packetswitched network to a first threshold, comparing, prior to establishingthe communication, the second resource availability through the circuitswitched network to a second threshold, storing a policy rule thatincludes a selection criteria between the packet switched network andthe circuit switched network, and transmitting the message through oneof the packet switched network or the circuit switched network based onapplying the policy rule to results of comparing the first resourceavailability through the packet switched network to the first thresholdand comparing the second resource availability through the circuitswitched network to the second threshold.
 8. The method of claim 7,where transmitting the message includes: randomly transmitting, based onthe policy rule, the message to one of the packet switched network orthe circuit switched network when the first resource availability,through the packet switched network, exceeds the first threshold and thesecond resource availability, through the circuit switched network,exceeds the second threshold.
 9. The method of claim 7, wheretransmitting the message includes: transmitting, based on the policyrule, the message to a predetermined one of the packet switched networkor the circuit switched network when the first resource availability,through the packet switched network, exceeds the first threshold and thesecond resource availability, through the circuit switched network,exceeds the second threshold.
 10. A system comprising: an InternetProtocol Multimedia Subsystem (IMS) device to: store a policy rule,receive a non-priority message, route the non-priority message throughone of a packet switched network or a circuit switched network based ona destination for the non-priority message, where the IMS device routesthe non-priority message without determining a first resourceavailability level through the packet switched network and a secondresource availability level through the circuit switched network, wherethe packet switched network includes a core network, associated with theIMS, and an access network that provides access to the core networkreceive a priority message, determine, in response to receiving thepriority message, the first resource availability level through thepacket switched network and the second resource availability levelthrough the circuit switched network, where the IMS device, whendetermining the first resource availability, is further to: determine,via a resource access control function (RACF) associated with the IMS,resource availability and topology information for the access network,determine, via the RACF, resource availability and topology informationfor the core network, and determining the first resource availabilitybased on the resource availability and topology information for theaccess network and the resource availability and topology informationfor the core network, and route the priority message through one of thepacket switched network or the circuit switched network based onapplying the policy rule to the first resource availability level andthe second resource availability level.
 11. The system of claim 10,where the IMS device is further to distinguish the non-priority messageand the priority message based on information in respective headers ofthe non-priority message and the priority message.
 12. The system ofclaim 10, where the priority message includes a Session InitiationProtocol (SIP) message, and where the IMS device identifies the prioritymessage based on a Resource-Priority header field of the SIP message.13. The system of claim 10, where, when the IMS device determines thefirst resource availability level, the IMS device receives resourceavailability information from a first device associated with the packetswitched network, where, when the IMS device determines the secondresource availability level, the IMS device receives resourceavailability information from a second device associated with thecircuit switched network.
 14. The system of claim 13, where the seconddevice includes a Breakout Gateway Control Function, and the firstdevice includes a Policy Decision Function associated with the RACF. 15.The system of claim 10, where the IMS device, when routing the prioritymessage, forwards the priority message to a Public Safety AnsweringPoint via the one of the packet switched network or the circuit switchednetwork.
 16. The system of claim 10, where the IMS, when forwarding thepriority message, is further to: compare, prior to routing the prioritymessage, the first resource availability level through the packetswitched network to a first threshold, compare, prior to routing thepriority message, the second resource availability level through thecircuit switched network to a second threshold, and forward the prioritymessage based on a result of comparing the first resource availabilitylevel through the packet switched network to the first threshold and aresult of comparing the second resource availability level through thecircuit switched network to the second threshold.
 17. The system ofclaim 16, where, when the IMS device forwards the priority message, theIMS device randomly forwards, based on the policy rule, the prioritymessage to one of the packet switched network or the circuit switchednetwork when the first resource availability level exceeds the firstthreshold and the second resource availability level exceeds the secondthreshold.
 18. A method comprising: receiving, in an Internet ProtocolMultimedia Subsystem (IMS) associated with the packet switched network,a Session Initiation Protocol (SIP) call request, where the packetswitched network includes a core network associated with the IMS and anaccess network that provides access to the core network; identifying,via the IMS, the SIP call request as one of an emergency call request oran non-emergency call request; routing, via the IMS and when the SIPcall request is identified as the emergency call request, the SIP callrequest toward a destination via one of a circuit switched network, apacket switched network, or a backup emergency link, where the circuitswitched network, the packet switched network, and the backup emergencylink differ, and where routing the SIP call request includes: obtaininga first resource utilization associated with the CIRCUIT SWITCHEDNETWORK, comparing the first resource utilization to a first thresholdto obtain a first comparison result, obtaining a second resourceutilization associated with the packet switched network, where obtainingthe second resource utilization includes: determining, via a resourceaccess control function (RACF) associated with the IMS, resourceavailability and topology information for the access network,determining, via the RACF, resource availability and topologyinformation for the core network, and determining the second resourceutilization based on the resource availability and topology informationfor the access network and the resource availability and topologyinformation for the core network, comparing the second resourceutilization, in the packet switched network, to a second threshold toobtain a second comparison result, and forwarding the SIP call requesttoward the destination via one of the circuit switched network, thepacket switched network, or the backup emergency link based on the firstcomparison result and the second comparison result; and routing, via theIMS and when the SIP call request is identified as the non-emergencycall request, the SIP call request toward the destination withoutcomparing the first resource utilization to the first threshold andwithout comparing the second resource utilization to the secondthreshold.
 19. The method of claim 18, where the first threshold and thesecond threshold are configurable by a user.
 20. The method of claim 18,where identifying the SIP call request as one of the emergency callrequest or the non-emergency call request includes: identifying the SIPcall request as the one of the emergency call request or thenon-emergency call request based on information stored in aResource-Priority header field of the SIP call request.
 21. The methodof claim 18, where forwarding the SIP call request includes: forwarding,via the backup emergency link, the SIP call request to the destinationwhen the first comparison result indicates that the first resourceutilization exceeds the first threshold, and the second comparisonresult indicates that the second resource utilization exceeds the secondthreshold.
 22. The method of claim 18, where forwarding the SIP callrequest via the backup emergency link includes: establishing, by aServing Call Session Control Function (S-CSCF) associated with the IMSand via the backup emergency link, a communication session on the backupemergency link, based on the SIP call request.